Jon Lester was knocked out by a line drive near his right knee Friday night, when Joba Chamberlain led New York over Boston 9-5 for his first win in 1½ months.

Alex Rodriguez homered, drove in four runs and tied a career high with three steals as the Yankees, already assured of a playoff berth, lowered to three the magic number for their first AL East title since 2006.

New York has won seven of eight against Boston, including a four-game sweep at Yankee Stadium from Aug. 6-9, after starting the season 0-8 against the Red Sox.

Rodriguez’s two-run homer in the third was New York’s team-record 127th at new Yankee Stadium. New York hit 126 homers at old Yankee Stadium in both 2004 and 2005, according to STATS LLC.

The Yankees also swiped seven bases in seven tries off Jason Varitek, their most steals since they had eight at Oakland on June 2, 1996.

Victor Martinez hit a solo shot for Boston, and David Ortiz added a two-run drive off Chamberlain and had three RBIs. Ortiz has 27 homers, all but one from June 6 on.

The crowd of 48,449 was just the sixth sellout in 76 games at new Yankee Stadium — the others were on opening day and during the August sweep of the Red Sox.

Lester (14-8), Boston’s best starter during the second half of the season, crumpled to the mound in pain when struck by Melky Cabrera’s liner in the third inning. His head back and eyes closed, Lester was looked over by trainers from both teams. After about 2 minutes, Lester hobbled off with the assistance of Red Sox trainer Paul Lessard, and Ortiz helped him down the dugout steps.

Boston said X-rays were negative and Lester had a bruised quadriceps. He allowed five runs and eight hits in 2 1-3 innings.

With New York 6½ games in front of the Red Sox, and Boston beginning the night eight games ahead of Texas in the wild-card race, there was little drama in the opener of the three-game series. New York could clinch the division over the weekend with a sweep, and Boston entered with a magic number of three for the wild card.

“If we’re fortunate enough to keep playing, when this is over, nobody’s going to care what happened,” Boston manager Terry Francona said before the game.

Chamberlain (9-6), slated to be the Yankees’ No. 4 starter in the AL championship series and World Series, was under scrutiny. The Yankees held down his pitch count and innings starting last month to keep the 23-year-old right-hander under his preset and undisclosed limit, and he had scuffled ever since — going 0-4 with 8.42 ERA in eight starts since beating Boston on Aug. 6.

He retired his first 11 batters before Martinez’s homer and allowed three runs and five hits in six innings, throwing 86 pitches. He struck out five and walked one.

“We need to see him pitch well,” manager Joe Girardi said before the game.

Chamberlain’s status has become a hot topic in New York, with some advocating he go back to the bullpen.

“Everyone seems to have the idea that they know what’s best for Joba. Our organization has put a lot of thought into this,” Girardi said before adding that some have overreacted. “It’s like he’s 1-19 with a 7 or 8 ERA. I mean, he hasn’t been the dominant guy that you would be for one inning, but no starter is a dominant guy for eight innings, like they are in a one-inning period over time.”

NOTES: The Yankees plan to start A.J. Burnett, Chad Gaudin and Chamberlain against Kansas City next week. That would enable them to flip flop Burnett and Andy Pettitte, with Burnett following CC Sabathia and starting Game 2 of the playoffs. ... Francona intended to start Varitek in two of three games over the weekend, but might start Martinez behind the plate when Daisuke Matsuzaka pitches Saturday. ... A-Rod has 93 RBIs. ... Martinez extended his hitting streak to 24 games. ... Panama President Ricardo Martinelli threw out the ceremonial first pitch. Wearing a No. 42 jersey of Mariano Rivera, he bounced the pitch to the Yankees’ Panamanian closer. ... Yankees RHP David Robertson, who hasn’t pitched in a game since Sept. 5 because of elbow stiffness, is to throw to hitters Saturday and could pitch in a game early next week. ... Jets QB Mark Sanchez was at the game. ... Some in the crowd booed former New York City Mayor Rudy Giuliani when he was shown on the video board during “God Bless America.”

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